The present invention relates to a color image forming apparatus by which a color image can be obtained in such a manner that: a toner image is formed on a belt-shaped image forming body by means of electrophotography; and the toner image is transferred onto a transfer sheet.
Numerous electrophotographic methods and apparatus for color image forming have been proposed heretofore. For instance, the following color image forming method has been disclosed in the official gazette of Japanese Patent Application Open to Public Inspection No. 100770/1986: latent images, the number of which corresponds to the number of separated colors of a document image, are formed and developed on a photoreceptor drum which functions as an image carrier. The developed image is transferred onto the surface of a transfer drum every time a latent image is developed so that a multicolor image can be obtained; and after transfer, the multicolor image is transferred onto a recording paper to obtain a color copy. In the color image forming apparatus described above, it is necessary to install not only a photoreceptor drum but also a transfer drum around the photoreceptor drum. Thus the circumferential surface of the transfer drum has to be wide enough to transfer an image sheet. Accordingly, the image forming apparatus is large and the structure is complicated.
Another color image forming method in which electrophotography is utilized, has been disclosed in the official gazette of Japanese Patent Application Open to Public Inspection 149972/1986, which is characterized in that: latent images, the number of which corresponds to the number of separated colors of a document image, are formed and developed on a photoreceptor drum; and the image is transferred onto a transfer sheet every time development is conducted. In this method, it is difficult to register the multicolor images with high accuracy, so that a color copy of high quality is hard to obtain.
Further another color image forming apparatus has been disclosed which is characterized in that: latent images, the number of which corresponds to the number of separated colors of a document image, are formed on a photoreceptor drum; the formed latent images are repeatedly developed by color toners so that color images can be registered on the photoreceptor drum; and the registered color toner images are transferred onto a recording paper in order to obtain a color image. The basic process of this multicolor image forming has been disclosed in the official gazettes of Japanese Patent Application Open to Public Inspection No. 75850/1985, 76766/1985, 95456/1985, 95458/1985, and 158475/1985, which were applied for by the present inventors.
In the color image forming apparatus in which a color image can be obtained by registering multicolor images, a plurality of developing units containing different color toners are provided around the photoreceptor drum, and the photoreceptor drum is usually rotated a plurality of times so that the color toner image on the photoreceptor drum can be developed in order to obtain a color image.
Regarding an image forming body, a belt-shaped image forming body in which a photoconductor is coated or fitted on a flexible belt has been proposed as well a& the above-described photoreceptor drum in which a photoconductor is coated or vapor-deposited on the drum surface. A belt-shaped image forming body, which will be called a photoreceptor belt hereinafter, is stretched between rollers including a drive roller, so that the space can be effectively utilized. Accordingly, utilizing a photoreceptor belt is an effective way to make a color image forming apparatus compact. Since the photoreceptor belt can be run along a small radius of curvature, a roller of small diameter can be used for the belt so that a transfer sheet can be separated from the belt at the position where the radius of curvature is small. In this way, the occurrence of separation failure of a transfer sheet can be prevented.
Regarding an image forming apparatus in which a process cartridge is utilized, which apparatus has been disclosed in the official gazettes of Japanese Patent Application Open to Public Inspection Nos. 244059/1988 and 244064/1988, the divided structure of transfer paper conveyance passage to remove a jammed transfer sheet has been proposed. In this case, however, the wiring of the fixing unit must be composed in such a manner that it can be divided, and the fixing unit must be composed so that it can be separated from the main body of the apparatus. Accordingly, this type of image forming apparatus is very disadvantageous to manufacture and operate.
In the case of this type of photoreceptor belt, the belt is horizontally stretched for the purpose of reducing the height of the apparatus, so that image forming means such as developing means and the like are provided below the photoreceptor belt.
The above-described photoreceptor bet and a group of developing units are formed into a unit and installed in the main body of the apparatus, ant further the paper feed cassette which contains recording papers is horizontally provided in the lower space. Therefore, in the case of image forming apparatus in which the photoreceptor belt is used, an apparatus has a paper feeding passage on the side of the image forming apparatus, which passage is vertically formed in such a manner that the recording paper is conveyed upward from the paper cassette located under the photoreceptor belt. Accordingly, in order to open the paper feeding passage, it is not sufficient to take out the above-described units. Since problems arise when removing jammed paper.
On the other hand, the lid of the above-described cassette chamber is provided on the side wall of the apparatus main body opposite to the side wall along which the above-described paper feeding passage and paper feeding means are provided, so that the paper feeding cassette can be inserted in the same direction as the paper feeding.
Consequently, there are caused the following disadvantageous in the above-described color image forming apparatus: other than the lid of the cassette chamber, a lid to remove a jammed paper from the paper feed passage needs to be provided on the apparatus main body side wall along which the above-described paper passage is provided; a large opening through which a cartridge is attached to and detached from the apparatus, in the case of image forming apparatus in which the photoreceptor belt and the developing units are integrally provided to form a cassette; and in the case of an image forming apparatus having the function of sheet by-pass, an opening to feed a paper manually needs to be provided. As a result, the structure of the apparatus becomes complicated and the production cost is increased. Further the operation and maintenance must be conducted over two surfaces, so that the working efficiency is decreased and furthermore a wide floor space is needed to operate and maintain the apparatus.
Recently, a color image forming apparatus in which a jammed paper can be easily removed and the cassette can be easily replaced have been disclosed in the official gazettes of Japanese Patent Application Open to Public Inspection Nos. 244058/1988, 244059/1988, 244064/1988 and 179168/1989. In this type of color image forming apparatus, the operation and maintenance can be conducted in the same direction. However, a color image forming apparatus which meets the requirements of users and service men such as replacing consumables quickly, replacing units easily and maintaining the apparatus efficiently, has not been developed heretofore.
Further, in the case of an image forming apparatus in which the paper feed cassette is inserted from the opposite side of the paper feed passage and the paper feed means, two side walls of the apparatus which are opposed to each other need to be opened in case a transfer paper jam has occurred, so that a wide space is needed to install the apparatus.
In order to overcome the disadvantage described above, an improved image forming apparatus has been proposed which is characterized in that: the cassette insertion opening is provided on the same main body side wall is the paper feeding passage; and all operation and maintenance work such as the insertion of the paper feeding cassette, the operation of each unit, the removal of a jammed paper and the maintenance work can be conducted on the front face of the apparatus.
In this type of prior art image forming apparatus, the fitted recording paper cassette protrudes from the apparatus main body, and the recording paper cassette can be easily attached to and detached from the apparatus main body by holding the protruding portion of the cassette.
However, there is a demand for compact image forming apparatus, so that the protruding paper cassette is not preferable because it takes a wide floor space.